This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Appln. No. 100 26 121.3 filed in Germany on May 26, 2000; the entire content of which is hereby incorporated by reference.
The present invention relates to an apparatus for damping acoustic vibrations in a combustor, as well as a combustor arrangement, in particular of a gas or steam turbine, that contains the apparatus.
The main field of application of the present invention is the field of industrial gas turbines. Worldwide, increasingly higher requirements with respect to readiness, life span, and waste gas quality are placed on industrial gas turbines, especially when used in power plants. An increasing consciousness of environmental protection and environmental compatibility requires compliance with the lowest possible values for noxious emissions.
Low emissions can only be achieved economically in industrial gas turbines by using premix burners. However, in closed combustors, because of the creation of coherent structures and resulting variable release of heat, this type of combustion tends to generate thermoacoustic vibrations in the combustor. These thermoacoustic vibrations do not only adversely affect the combustion quality, but also may drastically reduce the life span of the highly stressed components.
The principle of the so-called Helmholtz resonator has been used for a long time to dampen such thermoacoustic vibrations. This principle is explained in more detail below in reference to FIG. 1. The figure shows the principal structure of a Helmholtz resonator 4 comprising a resonance volume 3 and a connecting channel 2 to chamber 1, in which the thermoacoustic vibrations are occurring. Such an apparatus can be seen as analogous to a mechanical spring/mass system. The volume V of the Helmholtz resonator 4 hereby acts as a spring, and the gas present in the connecting channel 2 acts as the mass. The resonance frequency f0 of the system can be calculated using the volume dimensions:
whereby:
V=volume of Helmholtz resonator 4
R=radius of connecting channel 2
l=length of connecting channel 2
S=area of opening through which stimulation occurs
At this resonance frequency f0, a Helmholtz resonator behaves acoustically as an opening of infinite size, i.e., it prevents the creation of a standing wave at this frequency.
This technique of damping thermoacoustic vibrations with a Helmholtz resonator is also already used to dampen the vibrations in combustors of gas or steam turbines. However, when used in gas or steam turbines, the problem occurs that the frequency to be damped is not determined by intermittent combustion but by fulfilling the Rayleigh criterion in the combustor and by the acoustic response of the surrounding system comprising the supply line, burner, combustor, and acoustic terminus.
In these systems, the frequency to be damped therefore cannot be predetermined with the required accuracy by using the mathematical tools currently available. But this predetermination is the precondition for being able to take into consideration the exact dimensions of the resonance volume when building the gas turbine. Furthermore, the acoustical behavior of the system and thus the frequencies of the vibrations to be damped may critically change when the operating point is changed, so that it may become necessary to use additional resonators that are adapted to additional frequencies.
Such an arrangement with several Helmholtz resonators is described, for example, in DE 33 24 805 A1. This document concerns an apparatus for preventing pressure vibrations in combustors, in which apparatus several Helmholtz resonators with different resonance volumes are arranged along the gas conduit path towards the burner. The different resonator volumes in this system are able to dampen vibrations with different frequencies. However, here again the optimal sizing of the various Helmholtz resonators requires knowledge regarding the frequencies occurring during the operation of the system, where again exact frequencies cannot be provided when the system is being built. Furthermore, the arrangement of several Helmholtz resonators is disadvantageous due to the additional space needed for this purpose.
DE 196 40 980 A1 describes another known apparatus for damping thermoacoustic vibrations in a combustor. In this apparatus, the side wall of the resonance volume of the Helmholtz resonator is constructed as a mechanical spring. An additional mass has been secured to the wall of the front face of the resonance volume, said wall vibrating due to the action of the spring. This arrangement influences the virtual volume of the Helmholtz resonator and achieves greater damping power. By changing the mechanical mass at the resonator, a fine-tuning to the resonance frequency can be performed at a later time. This also requires a subsequent modification of the construction of the gas turbine system.
In the past, Helmholtz resonators were also used for damping vibrations in the field of exhaust gas systems of combustion engines. From this field, the use of adjustable resonators for changing the resonance frequencies is known. Even during World War I, for example, the two-cycle diesel engines for the Maybach company""s Zeppelin dirigible were adjusted to the necessary operating point with adjustable resonators located in the exhaust pipe. For this purpose, mechanical gears moved cylinders inside each other and in this way changed the resonance volume. In said exhaust systems, this technology was found to be practical because of the good accessibility of these systems and the relatively low pressure and temperature ratio present there. But such a solution is completely unsuitable for use in the pressure range found in modern industrial gas turbines. The passing of a mechanical gear through the pressure container of a gas turbine would inevitably cause leaks and result in intolerable losses. The temperature influences associated with industrial gas turbines also could only be compensated for with a very complex gear.
The present invention describes an apparatus for damping thermoacoustic vibrations as well as a combustor arrangement comprising this apparatus that enables continuous adaptation to the frequencies of the vibrations to be damped even under high pressure conditions as occur, for example, in gas turbines.
The apparatus includes a Helmholtz resonator with a connecting channel that is connected to the combustor, for example, the combustor of a gas turbine. In contrast to the known damping devices, the present apparatus is provided with a hollow body, the volume of which can be changed by adding or draining a fluid via a supply line, and which is arranged either within the Helmholtz resonator or is located adjacent to it in such a way that the resonance volume of the Helmholtz resonator changes when the volume of the hollow body changes.
When the adjustable-volume hollow body is located in the Helmholtz resonator, the resonance volume decreases when the hollow body is inflated via the supply line, for example with a gas. Correspondingly, the resonance volume of the Helmholtz resonator increases, when a certain amount of the gas is drained from the hollow body. The change in resonance volume in the known manner causes a change in the resonance frequency.
In this way, the resonance frequency of the Helmholtz resonator can be adapted at any time to the thermoacoustic vibration frequencies occurring in the chamber volume by a simple inflation or deflation of the hollow body. For this reason, an exact knowledge of the frequencies occurring during operation is no longer necessary when the system is built. The vibrations can be damped by means of a broad spectrum of individually set frequencies. In practical use, the resonance frequency of the built-in resonators can be changed at any time during the operation of the system in accordance with the current operating point by changing the resonance volume.
As a special advantage, the resonance volume of the Helmholtz resonator that is usually located inside the pressure container of the gas turbine can be changed without movable parts having to be passed through the wall of the pressure container. The supply line to the hollow body can be constructed as a rigid body and therefore can be easily passed through the pressure container to the outside with a high degree of tightness.
In another embodiment of the present invention, the Helmholtz resonator is provided with a variable-position wall, next to which the hollow body is located. The variable-position wall is pressed against the hollow body by a spring mechanism. In this way, the variable-position wall is pressed inward against the spring force when the hollow body is inflated and in this way reduces the resonance volume of the Helmholtz resonator. In the reverse case, the draining of gas from the hollow body causes the resonance volume to increase because the wall is shifted due to the spring force acting in the direction of the hollow body. The Helmholtz resonator hereby can be constructed in the form of a bellows, as is known from DE 196 40 980 A1, mentioned above. Naturally, it should be understood that other designs of the Helmholtz resonator are possible to achieve the effect described above.
In this embodiment, the variable-volume hollow body must be fixed at a point relative to the Helmholtz resonator within the pressure container in order to exert the corresponding counter-force onto the variable-position wall of the Helmholtz resonator.
The variable-volume hollow body is preferably constructed as an inflatable, temperature-resistant balloon or inflatable metal bellows. The supply line to the hollow body can be flexible or rigid.
In a preferred embodiment, the gas supply to the hollow body or the draining of gas from the hollow body is performed automatically by a regulator provided outside the pressure container on the supply line. This regulator changes the resonance volume of the Helmholtz resonator as a function of the highest amplitude frequency of the thermoacoustic vibrations occurring in the combustor by blowing the gas into the hollow body or draining it out. The respective vibration amplitudes and vibration frequencies are hereby measured with a suitable sensor, as known to one skilled in the art. The regulator preferably controls the resonance volume or volume of the hollow body by adding or draining compressor air received from the compressor outlet of the gas turbine. This makes it possible to achieve an optimum vibration damping at any time during the operation of the gas turbine, since the regulator is able to adapt the resonance volume at any time exactly to the currently occurring frequencies.